The Antichrist and/or Satan

In the Diabology of Pope Gregory the Great, Lucifer was the first to be created, a Cherub, highest of all angels, next only to God cast out of heaven for the sin of pride and thrown down into Hell as a fallen angel, where he became its king.

"They, in heaven, will condescend [to me], for I will exalt my throne above the stars of God"

The belief in Satan grew out of a belief in dualism, the eternal battle between Good and Evil in religion. and was the subject of many heresies in the early Church, which the church regularised, and after which Satan joined the pantheon of the Church, playing a major role in the faith and its theology.

There are many references to Satan, Lucifer, Belial or Baal, and the Antichrist in the medieval chronicles, correspondence and letters, and hagiographies of Becket, where he is sometimes called the Ancient Enemy. He has many names; he is also known as the Father of Lies, the Evil One, Beelzebub, Serpent of Old and so forth. Those affected by Satan had the mark of the beast branded on their forehead.

The father had barely made a month's stay in his church, when four knights came to Canterbury on the fifth day after the Nativity of our Lord [29th Dec], on the contrary the conspiring guards of Satan, men of pre-eminent distinguished families, but the irregular warfare, crime, adventure, fame and family a lasting shame to just condemn.

Hardly had the father been residing one month in his see, when lo! on the fifth day of the feast of the Nativity of our Lord, there came to Canterbury four knights, or rather sworn satellites [servants and followers] of Satan, whose names were as follows: William de Tracy, Hugh de Morville, Richard Brito, and Reginald Fitz-Urse, men of families remarkable for their respectability, but destined, by their daring to commit so enormous a crime, to blemish the glories of knighthood and the honors of their ancestors with perpetual ignominy.On the Easter Sunday following the murder of Becket the Pope excommunicated the four assassins describing them as the "satellites of Satan"The Antichrist and SatanEgbert Türk (1976). NUGAE CURIALIUM: LE REGNE D'HENRI II. Librairie Droz. pp. 21–2. ISBN978-2-600-03378-7.
During the absolution of Gilbert Foliot, bishop of London and the censure of Roger archbishop of York Becket described the former as the Anti-Christ and the latter as Satan.

After the pope absolved Foliot's excommunication in early 1170, Becket exclaimed to a cardinal that "Satan is unloosed for the destruction of the Church".

Thomas therefore refused to lay aside his safeguard, saying, If the King's sword slay the body, my sword strikes the spirit and sends the soul to Hell '; and when Roger still persisted in his entreaty, he detected the guile in his tongue. 'Get thee hence, Satan he replied; and Roger followed the rest of the bishops into the inner room where the King was.In 1170 Thomas Becket wrote to pope Alexander III on behalf of the poor brethren of Newburgh who had been oppressed, he said, by Roger Pont l'Eveque, the Lucifer who has established his seat in the North.

The Most High has himself has raised up Lucifer, putting his sacred edifice in the north, in the same place of the many-sided Christ, because he has taken possession of it, in the spirit and power of Elias, breaking the hammer of the whole earth.

The Council of Clarendon (Jan 1164) described as the Synagogue of Satan which Becket has joined by consenting to the Constitutions.William Holden Hutton (2014).Thomas Becket. Cambridge University Press. pp. 101–.ISBN978-1-107-66171-4.

and also

He turned his back upon the court at the close of another eventful day, and went in the direction of Winchester. Contrary to his usual habit, he rode alone, apart from his suite, in deep meditation. As they rode on, his attendants began to talk in a loud voice amongst themselves on the events of the day. Some said that what the Saint had done was necessary on account of the grave character of the time; others were indignant that the liberties of the Church should be at one man's beck. Alexander Llewellen, who carried the archiepiscopal cross, spoke up louder, to the alarm of the rest. " Public power disturbs every thing. Iniquity rages against Christ. The synagogue of Satan profanes God's sanctuary. The princes have sat, and united together against Christ. No one is safe who loves the truth. In the world's judgment they only are wise and venerated who blindly follow the king. This tempest has overthrown the columns of the Church; and during the shepherd's folly, the sheep are scattered before the wolf. Now that the chief has fallen, where will innocence be? who will stand? who will triumph in the battle?'' "To whom does this apply, my son?" said the Archbishop. " What virtue has he retained," he replied, " who has betrayed his conscience and his fame? It applies to you, who have to-day betrayed your conscience and your fame; and in an example left to posterity, which is hateful to God and contrary to justice, you have stretched out your consecrated hands to observe impious constitutions, and you have joined with wicked ministers of Satan to the overthrow of the liberty of the Church."

The Ancient Enemy

David Charles Douglas. English Historical Documents, 1042-1189. No. 122: Psychology Press. pp. 827–. ISBN978-0-415-14367-7.
Materials for the History of Thomas Becket Volume III, p.41.The ancient enemy was envious of so erect a pillar in the Church of the Lord, of so bright a candle on God's candlestick.

Furthermore, if we [Becket] fell at Clarendon, for the flesh is weak, we must take courage, and in the strength of the Holy Ghost contend against the ancient enemy, who is ever striving to make him fall who stands, and to prevent him from rising who has fallen. If, then.in the word of truth, we swore to what was unjust, you know that an unlawful oath is not binding.

The Bishops, being exempted from joining in the judgment, sat apart. In a short time the barons appeared, leaving but a very few of their number with the King. St. Thomas was about to rise to them as they entered; but Herbert whispered to him, that to receive them sitting would impress them with a deeper sense of the truth that they were judging their father, and would become him better who was carrying his cross. The Archbishop remained quiet, and gave no sign of fear on their drawing near.

The Spanish bishops were concerned to deny dualism categorically, especially the following ideas: that the Devil was independent of God rather than created by God as a good angel; that the Devil is creator of this world or introduces eveil things into it without god's permission; that the Devil is the creator of the hman body; that the conception of a child is the work of demons in the mother's womb/ The material universe, including the human body, the bishop's insisted, is the work, not of the malignant angel, but of God.

If
any one says the first human souls in the heavenly the habitation of
sin, and for this purpose in the bodies of the human in the land of the
ground, while he said, just as Priscillianus said, let him be anathema.

If anyone believes that the devil was not at first a good angel created by God, and that his nature was not the work of God, but if he claims that he emanated from chaos and darkness and had no author of his being, but that he is himself the principle and substance of evil, as stated by Manicheus and Priscillianus, let him be anathema.

If anyone belives that some unclean creatures [and not God] created the devil, and that the devil himself commands the thunders, lightnings, tempests and droughts, as Priscillianus has said, let him be anathema.

He who says that the creation of the human body is the work of the devil and that the said demon works on conceptions in the wombs of mothers, and on account of which believes not in the resurrection of the body (flesh) [of Christ], as Manicheus and Priscillianus have said, let him be anathema.